A CMOS image sensor uses MOS transistors as light sensitive pixels according to CMOS technology design rules. Around the periphery of the light sensitive region, the sensor integrates a control circuit and a signal processing circuit to sequentially detect outputs using the MOS transistors. A CMOS image sensor is therefore more conveniently driven in various scanning modes. Since a signal processing circuit can be integrated on a single chip in the CMOS image sensor, end products may be made smaller and less expensive, owing to the compatibility of the CMOS technology. Moreover, since power consumption of the CMOS image sensor is considerably smaller than that of a CCD, the CMOS image sensor may be used in more products.
FIG. 1 is a circuit diagram of a unit pixel of a 4-T CMOS image sensor. Referring to FIG. 1, a unit pixel of a 4-T CMOS image sensor includes a photodiode (PD) as a photosensing means and four NMOS transistors Tx, Rx, Dx, and Sx. The transfer transistor Tx delivers photo generated charges from the photodiode (PD) to a floating sensing node. The reset transistor Rx discharges charges stored in the floating sensing node for signal detection. The drive transistor Dx plays a role as a source follower. The select transistor Sx is provided for switching and addressing. A DC gate is a load transistor enabling a constant current to flow by applying a constant voltage as a gate potential of transistor. ‘VDD’ indicates a drive power source voltage and ‘VSS’ or “GND” indicates a ground voltage.
In a 4T CMOS image sensor, as the gate electrode of the transfer transistor Tx is turned on, electrons generated by the photodiode PD migrate into a floating diffusion (FD) area. In the floating diffusion area (FD), electrons migrating from the photodiode are transformed into a voltage using a source follower transistor. So, photo-energy is delivered as a voltage signal. In doing so, the floating diffusion area (FD) works as a sort of a capacitor. If the electrons are lost by leakage from the floating diffusion area (FD), the device is unable to deliver an accurate voltage. Hence, device performance is degraded.